Breaks build on libjava and tests in boehm-gc since
GC_register_my_thread, GC_unregister_my_thread, GC_get_thread_stack_base
are only implemented in pthread_support.c, they result in unresulved
symbols when one links agains libgcjgc.so.

Solaris instead uses solaris_threads.c.
Now we have either to reduce this patch to only support pthreads or
implement it for solaris_threads.c too.